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Applying Rack and Pinion in Linear Drive Systems

Mechanisms that generate linear motion are ranked by their positioning accuracy over distance, acceleration, axial force, stiffness and lifetime. Precision ground mounting surfaces, wear resistance surface finishes, optimized tooth profiles and low mass designs are some of the reasons why rack and pinion systems continue to be the first choice in applications with long travel distances.

When combined with efficient, low backlash gearboxes, rack and pinion systems deliver high dynamic performance, high repeatability and smooth motion. Precision hardened and ground racks can deliver pitch accuracies of < 0.012 mm per meter thanks to advantages in machining and grinding technology. This level of pitch error allows several pieces of rack to be joined together over long distances and can also allow machines to run without external feedback devices while other linear mechanisms require them for positioning. It is important to note that the quality of a rack is not only the deviation per meter, but also the heat treatment and hardening process.

Rack and pinion systems can be pre-loaded to completely eliminate backlash and increase system stiffness. This can be done accomplished mechanically by running a master/slave split pinion system where one pinion half drives the axis while the second pinion half is preloaded to remove backlash, via an axial spring pack at the end of the shaft. Electronical preloading utilizes dual servomotor/gearbox drives with pinions actively driving against the rack teeth in opposite directions. The preload is generated electronically with a motor controller.

Rack and pinion systems can be pre-loaded to completely eliminate backlash and increase system stiffness. This can be done accomplished mechanically by running a master/slave split pinion system where one pinion half drives the axis while the second pinion half is preloaded to remove backlash, via an axial spring pack at the end of the shaft. Electronical preloading utilizes dual servomotor/gearbox drives with pinions actively driving against the rack teeth in opposite directions. The preload is generated electronically with a motor controller. Automatic lubrication systems allow rack and pinion to last long and reach the highest speeds. Various sizes, volumes and flow rates are available, which can be directly controlled by PLC. The grease is pressurized and travels through a hose into a felt greasing pinion which applies the it onto either the rack or the pinion teeth.

Advantages over ball screws

Rack and pinion has lower moment of inertia, higher efficiency, higher stiffness and higher acceleration compared to ball screws. Length is a limitation of ball screws whereas racks can be infinitely adjoined. Ball screws will generally run up high cumulative errors over their travel length whereas rack and pinion can go as low as 12 µm if precision components are selected.

The higher moment of inertia that comes with ball screw systems will limit speeds and thrust load capability. Ball screws also rely on more adjacent components such as bearings and nut housings to maintain accuracy and stability.

Advantages over linear motors

Linear motors offer high performance and acceleration rates, but also come at a higher cost due to the materials used. Linear motors will often require water cooling, external housings, external brakes and linear scales which also add to system cost.

Linear motors are 85-90% efficient compared to up to 97% for rack and pinion drives. Higher energy consumption means larger investments in plant infrastructure. Linear motors must be free from metal particles due to magnetic attraction which can make them more challenging to maintain long term.

Sizing and selection

  • Tangential force (N): the force needed to deliver the linear movement profile.
  • Torque (Nm): Tangential force multiplied by the pinion radius. This is what the pinion sees.
  • Safety factor: Nidec DTC recommends x2 for horizontal drives and x3 for vertical drives.
  • Friction coefficient (μ): How heavy or light does the system run? Common values are 0.1 or 0.15.
  • External forces: Are there additional forces acting on the system, such as a machine tool cutting head? If so, these forces should be added to the tangential force.

ServoSoft is an invaluable sizing software that takes all of these factors and more into account. ServoSoft calculates linear drive systems based on user inputs and chooses the optimum combination of motor, gearbox, pinion and rack components. Nidec DTC gear reducers, rack and pinion products are included, along with a plethora of servomotor manufacturers, to save customers time. Our application engineering team can help you size up the mechanics and add a lubrication system.

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